1. Field of the Invention
This invention relates to a flexible printed wiring board, and more particularly, to a flexible printed wiring board having an element mounting part where circuit elements are mounted and a bending part to be bent around a bending axis.
2. Discussion of the Related Art
Recent progress in information society invites rapid increase in information quantity, and rapid exchange and transmission of a large capacity of information data are required. Consequently, integration in electronic circuit elements has been improved, and enhancement in performance, enhancement in function and enhancement in integration are in progress for electronic information apparatus. In such a trend, printed wiring boards used in electronic information apparatus are also undergoing enhancement in thinning, miniaturization, and intensified function, and various proposals have been made for flexible printed wiring boards as well. See Patent Document Nos. 1-5, listed below, for example.
Patent Document No. 1: Japanese Patent Laid-Open No. 1993-243741
Patent Document No. 2: Japanese Patent Laid-Open No. 1994-216537
Patent Document No. 3: Japanese Patent Laid-Open No. 1996-130351
Patent Document No. 4: Japanese Patent Laid-Open No. 1996-125342
Patent Document No. 5: Japanese Patent Laid-Open No. 1995-202358
Among such flexible printed wiring boards, with respect to those with memory elements mounted thereon, a new structure with an increased memory capacity and speed has been proposed and put into practical use. For example, flexible printed wiring boards in which a chip size package (CSP) is mountable to both surfaces have been developed.
Such a flexible printed wiring board with CSPs mounted on both surfaces is arranged to have a layered structure throughout the substrate surface, as shown in FIGS. 15A and 15B, typically consisting of three conductor layers (PT1 to PT3), two insulating layers (IN1 and IN2) isolating these conductor layers, and two coverlay layers (CL1 and CL2) (hereafter referred to as “related art example”). In addition, in a flexible printed wiring board of the related art example, memory elements, for example, are mounted on both surfaces of an element mounting part 60′ as shown in FIG. 15A. A bending part 70′ is bent around the bending axis AX′, so that a motherboard connecting part 80′, which is formed at the top surface on the other end, is made in electric contact with the motherboard MB (FIG. 15B). Consequently, the efficiency in implementation of memory elements on a motherboard can be improved.
When the above described related art example of a flexible printed wiring board is bent along the bending axis, tensile stress is applied to the outer side, while compressive stress is applied to the inner side. In addition, depending on the curvature at the time of bending, cracks may occur in the insulating layers. Therefore, it is preferable to decrease the curvature of bending; but the decrease in the curvature of bending would impede high-density mounting of memory elements.
Therefore, currently, a flexible printed wiring board with improved bending-withstanding properties is desired.